1. Field of the Invention
The present invention relates to an applicator for direct or indirect application of a liquid or pasty coating medium onto a traveling material web, notably of paper or cardboard.
2. Description of the Related Art
Applicators of the type described above are used in conjunction with so-called coaters to provide one or both sides of a traveling material web consisting, e.g., of paper, cardboard or textile material, with one or several layers of a coating medium, for example, dye, starch, impregnating fluid or the like.
In the so-called direct application, the liquid or pasty coating medium is applied by an applicating apparatus directly onto the surface of the traveling material web, which during application is carried on a revolving backing surface, for example, an endless belt or a backing roll. In the indirect application of the medium, in contrast, the liquid or pasty applicating medium, is first applied onto a substrate surface, e.g., the surface of a backing roll fashioned as an applicator roll, and is transferred there, in a nip through which the material is passed, from the applicator roll to the material web.
Known from U.S. Pat. No. 5,435,847 is an applicator with an applicating apparatus featuring a slot of constant width formed between two opposing walls, through which slot the coating medium being applied is passed onto the material web under pressure. The material web is usually carried by a backing roll associated with the applicating apparatus. This applicator additionally includes a coating edge delimiting apparatus with two coating edge delimiting elements in the form of deckle slides disposed in the slot between the opposing walls and sealing the slot on its side edges. The deckle slides are adapted to the constant width of the slot and have an essentially rectangular cross-sectional shape. Moreover, each of the deckle slides for adjustment of the coating width on the traveling material web is displaceable and fixable in the slot in a direction transverse to the direction of travel of the material web and remains during the operation of the applicator in the position once set. This prior applicator involves disadvantages to the effect that the liquid or pasty coating medium discharging from the slot often issues sideways or splashes out, depending on certain applicating or coating conditions, producing on the coated material web an untidy edge, or edge pattern, thus impairing the quality of the finished product.
Known from U.S. Pat. No. 5,436,030, furthermore, is an applicator for direct or indirect application of a liquid or pasty coating medium onto a traveling material web, notably of paper or cardboard, which applicator includes a dosing apparatus with a dosing slot fashioned as an open-jet nozzle and formed between lips on each of the approach side and the departure side. One embodiment features a deflection surface for the liquid or pasty coating medium, on the free end of the approach side or departure side. The deflection surface borders on the dosing slot and has a concave curvature. Of the two lips forming the dosing slot, the first lip, disposed on the side of the dosing slot that is approached by the applicator roll in the case of indirect application of the medium, or by the material web in the case of direct application, is termed the approach-side lip. Accordingly, the second lip, disposed on the side of the dosing slot from which the applicator roll or the material web departs from the applicator, is called the departure-side lip. In using a concavely curved deflection surface for the liquid or pasty coating medium, which borders on the dosing slot, it has been found that, depending on certain coating conditions or on the liquid or pasty coating medium used, so-called xe2x80x9ccolor sailsxe2x80x9d can form on the concave deflection surface. A color sail is a thin film developing from coating medium noses that form on the side edge of the dosing slot exit, and can possibly lead to an untidy coating edge.
An applicator comparable with that according to U.S. Pat. No. 5,436,030 is being marketed by the assignee of the present invention under the trade name xe2x80x9cJet Flow F.xe2x80x9d
Underlying the present invention, therefore, is the objective of improving a categorial applicator, i.e., an applicator similar to the previously described xe2x80x9cJet Flow F,xe2x80x9d to the effect that a high quality application with clean coating edges is achieved even under widely varying coating conditions.
The first inventional applicator for direct or indirect application of a liquid or pasty coating medium onto a traveling material web, notably of paper or cardboard, includes at least one dosing apparatus with a dosing slot fashioned as an open-jet nozzle and formed between two lips, of which, viewed in the flow direction of the coating medium, one is longer than the other and forms a guide surface for the coating medium. The applicator also includes a coating edge delimiting apparatus with at least two coating edge delimiting elements that are arranged between the two lips and close the dosing slot off on its side edges, substantially sealing it. Bordering on the end of the coating edge delimiting element associated with the dosing slot exit, viewed in cross section, is a wall section extending along the guide surface of the longer one of the two lips, essentially up to the free end of the longer lip.
Hence, an inventional coating edge delimiting element, the cross-sectional shape of which is preferably adapted to the cross-sectional shape of the dosing slot, protrudes with the wall section through the dosing slot, while the remaining sections of the coating edge delimiting element extend in an edge area situated below the dosing slot exit between the two lips in the dosing slot. The exact shape of the wall section of a coating edge delimiting element is adapted to the guide surface shape of the longer lip, so that the said wall section of the coating edge delimiting element fits flush and without clearance in planar fashion on the guide surface. The top edge of the wall section of a coating edge delimiting element ends on the free top edge of the guide surface of the longer one of the two lips, or somewhat below it. In the applicator according to the invention, the two lips between which the dosing slot is formed may be movable relative to each other, for adjustment of the dosing slot width. Moreover, the guide surface of the longer one of the two lips may be adjustable, for example, through the intermediary of a pivot axle or the like provided in the area of the dosing slot axis, for adjusting the angle of impingement of the coating medium flowing down the guide surface.
The inventional applicator avoids the disadvantages associated with the prior art and enables, even with widely varying coating conditions and widely varying coating medium types, a high-quality application with clean edges and a sharp definition between a coated and uncoated section of the material web, or of the applicator roll. Owing to the coating edge delimiting element of inventional design with a wall section extending from the dosing slot exit up to the top, free end of the guide surface of the longer lip, the liquid or pasty coating medium exiting from the dosing slot is presented, on the side edges of the dosing slot and beyond, an additional lateral guide surface formed by the coating edge delimiting element inside surface facing toward the dosing slot. The coating medium flows along the guide surface until it leaves the dosing apparatus on the top, free end of the guide surface and directly in front of the surface to be coated. A lateral discharge or splashing of the coating medium issuing out of the dosing slot is thus avoided in an easy and effective manner.
In one embodiment of the invention, the top end of the coating edge delimiting element wall section extends, viewed in the flow direction of the coating medium, beyond the free end of the longer lip. Thus, a lateral guide surface is presented to the coating medium even after leaving the guide surface of the longer one of the two lips, which lateral guide surface allows producing a sharp, clean coating edge.
The guide surface of the longer lip can be substantially straight or of a concave or convex shape. Appropriate mixed forms are conceivable as well.
In another embodiment feature of the invention, the wall section of a coating edge delimiting element has in the area of the guide surface a substantially constant wall thickness. This allows the wall section to adapt especially well to the respective shape of the guide surface of the longer lip. With the wall thickness of the coating edge delimiting element matching the dosing slot width, the coating edge delimiting element can, due to this dimensioning, also be easily pulled down and through the dosing slot for removal or replacement. But the invention, naturally, is not limited to this embodiment.
According to yet another embodiment of the inventional applicator, a coating edge delimiting element has a substantially flat inside surface that faces toward a center section of the dosing slot and, as explained above, serves as a lateral guide surface for the liquid or pasty coating medium. Such coating edge delimiting element can be fabricated with particular ease and achieves in most applications the advantages illustrated above.
In still another embodiment, a coating edge delimiting element to has a gutter-like inside surface facing toward a center section of the dosing slot. This allows a further enhancement of the guide effect of the inside surface, and produces good results with more problematic coating mediums.
The opposing coating edge delimiting elements are in the simplest design aligned on one another such that their inside surfaces facing toward the center section of the dosing slot extend substantially parallel to each other. In a further embodiment, the inside surface of a coating edge delimiting element facing toward the center section of the dosing slot has in a front view of the coating edge delimiting element, an arcuate edge pattern such that the inside surfaces of opposing coating edge delimiting elements have a converging arrangement. The liquid or pasty medium flowing along the coating edge delimiting element inside surfaces is thus additionally imparted an impulse directed inward, i.e., toward a center section of the guide surface. The impulse counteracts an unfavorable sideways discharge or splashing of the coating medium. Instead of an arcuate edge pattern, a straight, but oblique edge pattern is possible, which also leads to the converging configuration described above.
In a still further embodiment of the invention, at least one coating edge delimiting element is equipped with a recess for receiving an aperture board that is provided with a plurality of defined through slots. Based on the flow direction of the liquid or pasty coating medium, this recess is provided in the coating edge delimiting element in front of the dosing slot exit. Hence, the coating edge delimiting elements according to the invention can also be employed also in conjunction with an aperture board of the above kind.
The coating edge delimiting elements of the applicator according to the invention can be made of a plastic material. Conventional coating edge delimiting elements, in contrast, are usually are made of a metal such as, for example, bronze.
In another embodiment of the invention, the coating edge delimiting elements are disposed substantially parallel to the longitudinal expanse of the dosing slot, i.e., transverse to the direction of travel of the material web. This allows easy adjustment of the required coating format. The adjustment may be continuous or stepwise. The coating edge delimiting elements are suitably fixable in a chosen aperture board position.
Lastly, in yet another embodiment of the invention, the coating edge delimiting element is arranged substantially parallel to the longitudinal expanse of the dosing slot, allowing a stepwise or oscillating movement. The applicator suitably includes in this context at least one drive connected to the coating edge delimiting elements. The drive advances the coating edge delimiting elements in a stepwise and/or oscillating manner and/or moves them as needed to set the desired coating width. The movement can also be effected manually. Furthermore, the applicator may feature at least one control and/or regulating system communicating with the drive. The movable arrangement of the coating edge delimiting elements allows in an easy and effective manner a considerable reduction and equalization of the local wear at the transitions between the coated and uncoated edge of a doctor element such as typically employed in applicators of the categorial type for leveling the applied medium. Thus, the service life of the doctor elements is increased. As compared to the prior art, this enables longer service intervals before the required replacement of the doctor element, thereby avoiding, in turn, longer cumulative downtime of the applicator and achieving lower overall operating costs. Since no pronounced local point of wear occurs on the doctor element in the inventional applicator, the doctor element is usable with a format adjustment, i.e., a coating width, modified relative to the original setting, without requiring a replacement of the doctor element and without a reduction in quality of the finished product. Also, with the transition points between the coated and uncoated edge being located not on the applicator roll, or backing roll, but on the material web itself, an abrupt thickness difference between a coated and uncoated section of the material web can be effectively avoided. Rather, the applicator according to the invention produces a soft or xe2x80x9cblurredxe2x80x9d transition between the coated and uncoated section of the material web, which also considerably facilitates the winding, unwinding or rewinding of the material web as well as handling the finished product.
In the second applicator according to the invention, the two lips end approximately at the same level. At least one coating edge delimiting element extends here beyond the free ends of both lips, thereby achieving a still better sealing effect. An illustration of the suitable embodiment of the second invention shall be omitted here, since these options correspond largely to those of the first invention.